The mechanism(s) underlying the dramatic reduction in the risk of coronary heart disease associated with ingestion of diets rich in Omega 3 fatty acids (Om3FA) are not known, nor has the question of whether Om3FA benefit patients with the common potent risk factor, hypercholesterolemia (HC), been examined systematically. We have observed in preliminary experiments in HC patients that Om3FA lower LDL, raise HDL2, and rduce the content of free cholesterol/lecithin ratio of the lipoproteins creating a gradient favoring the centripetal movement of cholesterol from cells to the liver. In this application, we propose to examine systematically mechanisms that underlie these and other possible antiatherogenic changes in lipoproteins: by first comparing both quantitatively and qualitatively the lipoprotein responses to two differing Om3FA preparations (ethyl esters, triglycerides), in patients with three defined forms of HC. We then plan to determine whether compositional changes we have already shown in lipoproteins modify their physicochemical characteristics (size, phase transition temperature, subpopulation distribution, fluidity), and functional properties (the LDL catabolic pathways, HDL's capacity to promote cholesterol efflux from cells and transfer cholesterol ester to other lipoproteins). Next, we will assess the effects of Om3FA on the composition of plasma neutral and phospholipids and their molecular species, and on four key regulatory enzymes in lipoprotein metabolism (lipoprotein lipase, hepatic lipase, LCAT and lysolecithin acyl transferase). Hep G2 cells will be employed to determine whether Om3FA inhibit hepatic apoprotein B and triglyceride synthesis. Since Om3FA are incorporated into membranes through exchange reactions and biogenesis, we propose to examine their effects on certain critical membrane functions such as 1) the capacity to metabolize LDL; 2) the activities of plasma membrane-bound enzymes and receptors; and 3) the efflux of free cholesterol from cultured cells. This comprehensive experimental approach should provide detailed new information about the biological actions of Om3FA on lipoprotein metabolism, crucial to understanding how they reduce cardiovascular risk, and yield practical insights as to whether dietary supplementation can benefit patients with HC.